A Spindt-type electrode and a carbon nanotube electrode (CNT) have been known as conventional electron emitting elements. Applications of such conventional electron emitting elements to, for example, the field of Field Emission Display (FED) have been studied. Such electron emitting elements are caused to emit electrons by tunnel effect resulting from formation of an intense electric field of approximately 1 GV/m that is produced by application of a voltage to a pointed section.
Conventionally, it has been desired that an electron emitting element works in the atmosphere, and there has been an idea of, for example, applying such an electron emitting element to a charging device or an electrostatic latent image forming device. As for an example of an electron emitting element using the Spindt-type electrode, an electron emitting element for forming an electrostatic latent image has been proposed (e.g. refer to Patent Literature 1). This electron emitting element is operated in the atmosphere so as to emit electrons into the atmosphere. This generates, by ionizing gas molecules, ions serving as electrically charged particles so that an electrostatic latent image is formed. Meanwhile, there a report has been made on a result of research in which an electron emitting element using the carbon nanotube electrode is operated in the atmosphere (e.g. refer to Non Patent Literature 1).
However, each of these two types of the electron emitting elements has an intense electric field in the vicinity of a surface of an electron emitting section. Accordingly, electrons emitted obtain a large amount of energy due to the electric field. This makes it easy to ionize gas molecules. However, cations generated in the ionization of the gas molecules are accelerated in a direction of a surface of the element due to the intense electric field and collide with the surface. This causes a problem of breakdown of the element due to sputtering. Further, ozone is generated before ions are generated, because oxygen in the atmosphere has dissociation energy that is lower than ionization energy. Ozone is harmful to human bodies, and oxidizes various substances because of its strong oxidizing power. This causes a problem in that members around the element are damaged. In order to prevent this problem, the members used around the electron emitting element are limited to members that have high resistance to ozone.
Meanwhile, an MIM (Metal Insulator Metal) type and an MIS (Metal Insulator Semiconductor) type have been known as other types of electron emitting elements. These electron emitting elements are surface-emission-type electron emitting elements which accelerate electrons by utilizing quantum size effect and an intense electric field in the element so that electrons are emitted from a flat surface of the element. These electron emitting elements do not require an intense electric field outside the elements, because the electrons which are accelerated inside the elements are emitted to the outside. Therefore, each of the MIM type and the MIS type electron emitting elements can overcome the problems such that (i) the element is broken down by the sputtering which occurs due to ionization of gas molecules and (ii) ozone is generated, in the Spindt-type, CNT type, and BN type electron emitting elements.
As an example of the MIS type electron emitting element that utilizes quantum size effect of a porous semiconductor (e.g. porous silicon) which is formed by anodizing a semiconductor, another electron emitting element is proposed (e.g. refer to Patent Literature 2). The another electron emitting element (i) accelerates electrons injected into the porous semiconductor by use of an electric field and (ii) causes the electrons to pass through a surface thin metal film by tunnel effect so that the electrons are emitted in vacuum. Further, the above electron emitting element by use of the porous semiconductor has a great advantage in that the element can be produced by an anodic oxidation which is a very easy and inexpensive production method.
In addition, there has been known a still another electron emitting element in which a layer of semiconductor particles or fine metal particles whose surfaces are covered with insulating layers is repeatedly formed (e.g. refer to Patent Literature 3).